1. Field of the Invention
This application relates to the self-repair of polymer and polymer composite structures. The disclosed method of achieving this is to design and incorporate smart, intelligent, and adaptive systems into the structures composed of adaptive materials such as dynamic modulus resins and composites. Such systems will allow for continuous health and performance monitoring, fast and decisive information processing to ensure that the system is “highly aware” of its current health status and unparalleled in its adaptability to damage. Currently, most state-of-the-art health monitoring technologies can only deliver the indication of damage to the human operator via visual display. An integrated system consisting of smart, adaptive, and intelligent components will enable an advanced system to sense and immediately recover from physical damage while informing the operator of the situation but not requiring a response.
2. Description of Related Art
Dynamic Elastic Modulus Resins (DMR) are resins whose elastic modulus changes with a change in temperature of the resin. One such DMR is shape memory polymer (SMP). Shape memory materials were first developed about twenty-five (25) years ago and have been the subject of commercial development in the last fifteen (15) years. Shape memory materials derive their name from their inherent ability to return to their original “memorized” shape after undergoing a shape deformation. There are principally two types of shape memory materials, shape memory alloys (SMAs) and shape memory polymers (SMPs).
SMAs and SMPs that have been pre-formed can be more easily deformed to a desired shape above their glass transition temperature (Tg). The SMA and SMP must remain below, or be quenched to below, the Tg while maintained in the desired shape to “lock” in the deformation. Once the deformation is locked in, the SMA, because of its crystalline network, and the SMP, because of its polymer network, cannot return to a relaxed state due to thermal barriers. The SMA and SMP will hold its deformed shape indefinitely until it is heated above its Tg, whereupon the SMA and SMP stored mechanical strain is released and the SMA and SMP returns to its pre-formed, or memory, state.
There are principally two types of plastics, thermoset resins and thermoplastic resins, each with its own set of unique characteristics. Thermoset resins, for example polyesters, are liquids that react with a catalyst to form a solid, and cannot be returned to their liquid state, and therefore, cannot be reshaped without destroying the polymer networks. Thermoplastics resins, for example PVC, are also liquids that become solids. But unlike thermoset resins, thermoplastics are softened by application of heat or other catalysts. Thermoplastics can be heated, reshaped, heated, and reshaped over and over.
SMPs used in the presently disclosed method and devices are unique thermosetting polymers that, unlike traditional thermosetting polymers, can be reshaped and formed to a great extent because of their shape memory nature and will not return to a liquid upon application of heat. Thus by creating a shape memory polymer that is also a thermosetting polymer, designers can utilize the beneficial properties of both thermosetting and thermoplastic resins while eliminating or reducing the unwanted properties. Such polymers are described in U.S. Pat. No. 6,759,481 issued to Tong, on Jul. 6, 2004 which is incorporated herein by reference. Other thermoset resins are seen in PCT Application No. PCT/US2006/062179, filed by Tong, et al on Dec. 15, 2006; and PCT Application No. PCT/US2005/015685 filed by Tong et al, on May 5, 2005 of which both applications are incorporated herein by reference.
Additionally DMRs and SMPs can self-heal through combination of shape memory effects and a reptation process. Reptation theory describes the snake-like large-scale motion of long-chain entangled polymers across an interface, involving interfacial bonding across a boundary. The diffusive motion of polymer segments across a boundary is increased at high temperatures in which long-chain polymers are embedded. Essentially this motion allows two polymer surfaces to bond together along their interface when placed in intimate contact above the Tg of the polymer. This phenomenon commonly referred to as “healing” is essentially the interfacial welding of two polymer surfaces through the inter-diffusion of the polymer by motions across the interface via chain reptation-type motions. This basic picture of reptation is by now experimentally verified and well-established in many contexts.
This form of healing is commonly studied when polymers are placed in contact above Tg. As samples are heated and expand, the crack surfaces come into intimate contact and healing progresses following the diffusion process described above, mending the crack. The Tong patents mentioned above can be formulated by those of skill in the art to include this self-healing feature.
There are three types of SMP's: 1) A partially cured resin, 2) thermoplastics, and 3) fully cured thermoset systems. There are limitations and drawbacks to the first two types of SMP. Partially cured resins continue to cure during operation and change properties with every cycle. Thermoplastic SMP “creeps,” which mean it gradually “forgets” its memory shape over time. A thorough understanding of the chemical mechanisms involved will allow those of skill in the art to tailor the formulations of SMP to meet specific needs, although generally fully cured thermoset resin systems are preferred in manufacturing.
While SMA and SMP appear to operate similarly on the macro scale, at the molecular scale it is apparent that the method of operation of each is very different. The difference between SMA and SMP at the molecular level is in the linkages between molecules. SMA essentially has fixed length linkages that exist at alternating angles establishing in a zigzag patterned molecular structure. Reshaping is achieved by straightening the angled connections from alternating angles to straight forming a cubic like structure. This method of reshaping SMA material enables bending while limiting any local strains within the SMA materials to less than eight percent (8%) strain, as the maximum shape memory strain for SMA is eight percent (8%). This eight percent (8%) strain allows for the expansion or contraction of the SMA by only 8%, a strain that is not useful for most industrial applications. Recovery to memory shape is achieved by heating the material above a certain temperature at which point the molecules return to their original zigzag molecular configuration with significant force thereby reestablishing the memory shape. The molecular change in SMA is considered a metallic phase change from Austensite to Martensite which is defined by the two different molecular structures.
SMP has connections between molecules with some slack. When heated these links between connections are easily contorted, stretched and reoriented due to their elastic nature as the SMP behaves like an elastic material when heated, when cooled, the shape is fixed to how it was being held. In the cooled state the material behaves as a typical rigid polymer that was manufactured in that shape. Once heated the material again returns to the elastic state and can be reformed or return to the memory shape with very low force. Unlike SMA which possesses two different molecular structures, SMP is either a soft elastomer when heated or a rigid polymer when cool. Both SMA and SMP can be formulated to adjust the activation temperature for various applications. Critical to the success of the currently claimed device is thermoset SMP which provides an order of magnitude higher stiffness than previous state-of-the-art thermoplastic SMPs. This added stiffness coupled with high strain capability enables the development and use of a highly useful composite tooling technology.
Unlike SMAs, SMPs exhibit a radical change from a normal rigid polymer to a flexible elastic and back on command. SMA would be more difficult to use for most applications because SMAs do not have the ease in changing the activation temperature as do SMP's. SMAs would also have issues with galvanic reactions with other metals which would lead to long term instability. The current supply chain for SMAs is currently not consistent as well. SMP materials offer the stability and availability of a plastic and are more inert than SMAs. Additionally, when made into a composite SMPs offer similar if not identical mechanical properties to that of traditional metals and SMAs in particular. Throughout this disclosure SMP and SMP composites are used interchangeably as each can be replaced by the other depending on the specific design requirements to be met.
The term “composite” is commonly used in industry to identify components produced by impregnating a fibrous material with a thermoplastic or thermosetting resin to form laminates or layers. Generally, polymers and polymer composites have the advantages of weight saving, high specific mechanical properties, and good corrosion resistance, which make them indispensable materials in all areas of manufacturing. Nevertheless, manufacturing costs are sometimes detrimental, since they can represent a considerable part of the total costs and are made even more costly by the inability to quickly and easily repair these materials without requiring a complete, and expensive, total replacement. Because SMPs are resins, they can be used to make composites, which are referred to in this application as SMP composites.
Advanced composites, containing continuous fibers dispersed in a resin matrix material, are widely used in aerospace, sports equipment, infrastructure, automotive, and other industries both as primary and secondary load-bearing structures. These composite materials derive their excellent mechanical strength, stiffness, and other properties from a combination of the resin and reinforcement fibers used. The addition of reinforcements such as continuous fiber, fiber mats, chopped fibers, fiberglass, nanoparticles and other similar material is known. Even with nanoparticles like carbon nanotubes and carbon nano-fillers a small amount of these nano-fillers could dramatically alter the properties of a matrix resin.
A recurring issue in product applications using materials such as polymeric materials is that they tend to fail or degrade due to mechanical fatigue, mechanical impact, oxidation due to radiation or impurities, thermal fatigue, chemical degradation, or a combination of these processes. The degradation can lead to embrittlement of the polymer along with other adverse effects. The embrittlement and associated cracking can advance to a point that it causes product failure and associated replacement costs. Thermoplastic and thermoset polymer systems used in products can be particularly susceptible to these failures.
This problem is of great concern because of the widespread and intensive use in modern society of polymers and polymer composites in product components. Traditional approaches to increasing the reliability of polymeric based components and products have included a focus on suitable design enhancements and the use of incrementally improved plastics.
One recently developed process to impart self-healing capability to a polymer involves the incorporation of microcapsules containing a healing agent in a polymer matrix. When a fracture occurs in the polymer matrix in close proximity to the microcapsules the associated stresses caused by the fracture ruptures the microcapsules. As a consequence the healing agent is released from the ruptured microcapsules and contacts the fracture surfaces. At the same time the healing agent comes into contact with a polymerization agent dispersed in the polymer matrix. The polymerization agent is functionally active in the presence of various chemicals including moisture in the air. When the polymerization agent contacts the self-healing agent and promotes polymerization of the healing agent resulting in filling the crack planes of the fracture.
U.S. Pat. No. 7,285,306 issued on Oct. 23, 2007 to Parrish discloses a self-healing system for an insulation material wherein the self repair process is initiated by rupturing a plurality of microcapsules disposed on the insulation material. When a plurality of microcapsules is ruptured, reactants within the plurality of microcapsules react to form a replacement polymer in a break of the insulation material.
U.S. Pat. No. 7,108,914 issued on Sep. 19, 2006 to Skipor et al. also discloses a self-healing polymer composition containing a polymer media and a plurality of microcapsules of flowable polymerizable material dispersed in the polymer media, where the microcapsules of flowable polymerizable material containing a flowable polymerizable material and have an outer surface upon which at least one polymerization agent is chemically attached. The microcapsules are effective for rupturing with a failure of the polymeric media and the flowable polymerizable material reacts with the polymerization agent when the polymerizable material makes contact with the polymerization agent upon rupture of the microcapsules.
The principal drawback of Parrish and Skipor is that once the microcapsules have ruptured and repaired the insulation a second break or damage point at or near the first break or damage point cannot be as easily repaired because the replacement polymers in the microcapsules will have been used in the first repair.
U.S. Pat. Nos. 6,261,360; 5,989,334; 5,660,624; 5,575,841; and 5,5611,73 issued to Dry describe a cured composite matrix having a plurality of hollow release vessels usually fibers dispersed therein with the hollow fibers having a selectively releasable modifying agent contained within them a means for maintaining and modifying agent within the fibers until selectively released and a means for permitting selective release of the modifying agent from the hollow fibers into the matrix material in response to at least one predetermined external stimulus. The cured matrix materials have within them fibers capable of delivering repair agents into the matrix wherever and whenever they are needed.
While this engineered healing composite represents a very exciting advance in the self-repair of materials, it is limited to crack-type damage and would not be expected to heal the large sized projectile damage (several mm or more in diameter) or repair damage at the same point multiple times. The biggest difference between these patent and the presently disclosed system is the fact that the presently disclosed system is known to heal via a thermo-mechanical response rather than by chemical reaction.
International Application No. PCT/US2005/0198 filed Jun. 6, 2005 describes a manual process to repair damage in a material thought the application of a SMP or SMP composite patch. The pressure sensitive adhesive placed on one side of the patch bonds the patch to the damaged area, covering the damage. The SMP in the patch allows a human operator to mold the patch to accurately fit the product being repaired. This method is most useful for aesthetic repairs to a product, not for structural repairs because the damage area will remain and could propagate beyond the boundaries of the patch at a future point. Additionally this device and method of repair requires a separate piece of SMP or SMP composite and a human operator to effect repairs.